Sintered bodies of .beta.-type sialon find use in varied application areas on account of their advantages such as high strengths at high temperatures, superior oxidation resistance, small coefficients of thermal expansion, and great thermal-shock resistance.
There is proposed a sialon containing 0.2-10 wt % of Y.sub.2 O.sub.3 and 10-40 wt % of TiN, which is intended to make a sintered body for cutting tools. (See Japanese Patent Laid-open No. 213678/1983.) There is also proposed a sialon containing 0.2-10 wt % of Y.sub.2 O.sub.3, 0.5-10 wt % of ZrO.sub.2, and 10-30 wt % of TiN, which is intended for the same application as above. (See Japanese Patent Laid-open No. 3073/1984.) These sialons contain TiN and 0.2-10 wt % of Y.sub.2 O.sub.3, which add high toughness and high hardness to the sialon sintered bodies which are used for cutting tools on account of their superior oxidation resistance and thermal-shock resistance. However, sialon sintered bodies containing Y.sub.2 O.sub.3 suffer from expansion and cracking at high temperatures (about 850.degree. C.). This is not disclosed in the above-cited literature.
A sintered body of sialon can be used as a heater element as disclosed in Japanese Patent Laid-open No. 33265/1985 filed by the present assignee. It is composed of 30-70 vol % of Si.sub.3 N.sub.4, 30-70 vol % of TiN (to impart conductivity), and 1-10 wt % of Al.sub.2 O.sub.3, Y.sub.2 O.sub.3, and MgO or AlN or both. It has a resistivity lower than 10.sup.-2 .OMEGA..multidot.cm at room temperature and a positive resistance-temperature coefficient. It contains 1-10 wt % of Al.sub.2 O.sub.3, Y.sub.2 O.sub.3, and MgO or AlN or both, which function as sintering auxiliaries, so that it has increased strength at 1000.degree. C. and above, and it also has a positive resistance-temperature coefficient. Nevertheless, nothing is taken into account about the formation and prevention of an oxide layer at high temperatures (about 850.degree. C.). In the meantime, Japanese Patent Laid-open No. 17264/1988 filed by the present assignee discloses a sintered body composed of 30- 75 vol % of .beta.-sialon phase, 25-70 vol % of conductive oxide, nitride, and carbide phases, and a grain boundary phase of Y.sub.2 O.sub.3. This sintered body requires that the conductive phase should have a specific particle diameter, so that it has good electrodischarge machinability, oxidation resistance, and thermalshock resistance. Nevertheless, nothing is mentioned about the fact that Y.sub.2 O.sub.3 is associated with the expansion and cracking that occur in the sintered body at high temperatures in the neighborhood of 850.degree. C.
There is disclosed a ceramics sintered body similar to a sialon sintered body in Japanese Patent Laid-open No. 41771/1983. It is composed of Si.sub.3 N.sub.4 powder, 0.05-5 vol % of one or more members of Y.sub.2 O.sub.3, Al.sub.2 O.sub.3, etc. as a sintering auxiliary, and 1-25 vol % of one or more kinds of Ti carbonitrides and the like impart the conductivity. This composition is intended to improve the electrical conductivity of the grain boundary layer, thereby improving the electrical conductivity of the entire sintered body and imparting electrical conductivity to the sintered body.
There is proposed a ceramics heater element of a sintered body composed of Si.sub.3 N.sub.4 (major constituent), 2.9-51.5 vol % of TiN, and 3.5-15 mol % of Y.sub.2 O.sub.3, MgO, Al.sub.2 O.sub.3, and SiO.sub.2. (See Japanese Patent Laid-open No. 60983/1985.) This sintered body is intended to have high strength, thermal-shock resistance and heat resistance at high temperatures in the neighborhood of 1200.degree. C.; however, nothing is disclosed about the function and effect which are produced by specifying the content of Y.sub.2 O.sub.3 as the sintering auxiliary.